1. Field of the Invention
The present invention relates to a display device having a light-emitting element such as an organic EL, and particularly to a technique by which temporal changes in characteristics of light emission of an organic EL element are detected.
2. Description of the Related Art
Although the mainstream of a conventional display device was a CRT, a liquid crystal display device, a plasma display device, and the like that are flat display devices are put to practical use in place of a CRT, and the demand is on the rise. Further, in addition to these display devices, a display device (hereinbelow, referred to as an organic EL display device (OLED)) using organic electroluminescence and a display device (hereinbelow, referred to as an FED display device) in which electron sources using field emission are arranged in a matrix manner and fluorescent materials arranged at anodes are lit so as to form an image have been being developed and put to practical use.
The organic EL display device has the following characteristics: (1) a backlight is not necessary because the organic EL display device is of a light-emitting type as compared to a liquid crystal display; (2) a voltage required for light emission is as low as 10V or less, and power consumption can be possibly reduced; (3) a vacuum structure is not necessary, so that the organic EL display device can be suitably made lighter and thinner as compared to the plasma display device and the FED display device; (4) response time is as short as several micro seconds, and the characteristics of a moving picture are excellent, and (5) a view angle is as wide as 170 degrees or more.
Although the organic EL display device has the above-described characteristics, one of the problems is that there is a phenomenon that the characteristics of light emission of an organic light emitting diode element (hereinbelow, referred to as an OLED element) are changed along with the operation time. The temporal changes in characteristics of the OLED element differ depending on pixels. Accordingly, in order to correctly display an image, it is necessary to detect the changes in characteristics of the OLED element of each pixel and to feed back the result to a signal input from a host.
The changes in characteristics of the OLED element are present as changes in voltage-current characteristics of the OLED element. Specifically, even when a voltage is applied at the same level, a current to flow is decreased along with the operation time. This phenomenon is shown in FIG. 11. The horizontal axis of FIG. 11 represents a voltage applied to the OLED element, and the vertical axis thereof represents a current flowing into the OLED element. The characteristic 1 represents an initial characteristic of the OLED element. The characteristic 2 represents a characteristic of the OLED element after a certain period of time passed. Since it is conceivable that light emission of the OLED element is in proportion to a current flowing into the OLED element, even when a voltage is applied at the same level, the brightness of light emission of the OLED element is changed along with the passage of time, and thus an image can not be correctly displayed.
Conversely, this means that it is necessary to apply a higher voltage in order to flow a current at the same level so as to emit light at the same level. FIG. 12 shows changes in applied voltage in order to flow a current at the same level into the OLED element. In FIG. 12, the horizontal axis represents an operation time and the vertical axis represents an applied voltage for flowing a constant current into the OLED element. FIG. 12 shows that in order to flow a current at the same level to the OLED element, an applied voltage needs to be increased along with the operation time.
As described above, in order to correctly display an image in the organic EL display device, it is necessary to regularly measure the voltage-current characteristics of the OLED elements of the all pixels and to feed back the result to an input image signal. JP-A-2005-156697 and JP-A-2002-341825 describe such techniques.
In all of the above-described conventional techniques, the OLED elements of the all pixels are sequentially measured. In the case where the voltage-current characteristics of the OLED elements of the respective pixels are measured, it is necessary to charge floating capacitance due to presence of the floating capacitance in the respective pixels. Accordingly, it takes time to measure each pixel. In addition, when the screen of the display device becomes larger and the definition of the screen becomes higher, it takes a lot of time to measure the all pixels.
When the measuring time becomes longer, a period during which an image is displayed is limited. However, it is necessary to maintain the practical brightness of the display, so that a current at a higher level needs to flow into the OLED elements during the display period, and thus there arise various problems such as voltage drop at a power source line.
On the other hand, it is conceivable to increase a current at the time of measuring in order to shorten the measuring time. However, flowing of a current at a higher level results in increase in size of a circuit for measuring and in range of a voltage used. However, the increase of a current is not preferable because the increase in size of the measuring system leads to increase in cost of the display device by the amount of the increased size. Further, the increase of a current for measuring leads to consumption of a large electric power for measuring. Thus, the increase of a current is not preferable from this aspect.